The invention relates generally to bearing assemblies, and in particular to bearing assemblies for downhole motors for use in drilling of oil, gas, and water wells.
In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or, in offshore drilling operations, on a seabed-supported drilling platform or a suitably adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the borehole, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall).
Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill string immediately above the drill bit. A typical downhole motor includes several primary components, as follows (in order, starting from the top of the motor assembly):                a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);        a power section comprising a positive displacement motor, with a helically-vaned rotor eccentrically rotatable within a stator section;        a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and        a bearing section comprising a cylindrical mandrel coaxially and rotatably disposed within a cylindrical housing, with an upper end coupled to the lower end of the drive shaft, and a lower end adapted for connection to a drill bit.        
In drilling processes using a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the downhole motor to generate power to rotate the drill bit.
The bearing section must permit relative rotation between the mandrel and the housing, while also transferring axial thrust loads between the mandrel and the housing. Axial thrust loads arise in two drilling operational modes: “on-bottom” loading, and “off-bottom” loading. On-bottom loading corresponds to the operational mode during which the drill bit is boring into a subsurface formation under vertical load from the weight of the drill string, which in turn is in compression; in other words, the drill bit is on the bottom of the borehole. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the borehole and the drill string is in tension. This condition occurs, for instance, when the drill string is being pulled out of the borehole, putting the drill string into tension due to the weight of drill string components. Tension loads across the bearing section housing and mandrel are also induced when circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly.
Accordingly, the bearing section of a downhole motor must be capable of withstanding thrust loads in both axial directions, with the mandrel rotating inside the housing. This has been accomplished in some prior art bearing assemblies by using a first thrust bearing assembly configured to resist on-bottom thrust loads only, and a second thrust bearing assembly configured to resist off-bottom thrust loads only. Suitable radial bearings are used to maintain coaxial alignment between the mandrel and the bearing housing.
Each thrust bearing assembly typically comprises bearings (usually but not necessarily roller bearings) contained within a bearing cage disposed within an annular bearing containment chamber, the outer portion of which extends radially outward into the housing, and the inner portion of which extends radially inward into the mandrel. Other prior art bearing sections for downhole motors use one or more bi-directional thrust bearings rather than separate thrust bearings for on-bottom and off-bottom loads. Using this arrangement, the bearing section can be significantly shorter than a bearing section incorporating separate on-bottom and off-bottom thrust bearing assemblies.
U.S. Pat. No. 5,150,972 (Wenzel) provides an example of a downhole motor bearing assembly that uses a bi-directional thrust bearing disposed within a bearing containment chamber generally as described above. The inner and outer portions of the containment chamber each define an upper shoulder and a lower shoulder. These four shoulders lie in parallel planes which are perpendicularly transverse to the coincident longitudinal axes of the housing and mandrel. The inner upper shoulder is closely adjacent to the outer upper shoulder, and the inner lower shoulder is closely adjacent to the outer lower shoulder. The relative axial positions of the inner and outer shoulders will change slightly due to relative axial movement between the housing and the mandrel as the assembly shifts between on-bottom and off-bottom loading conditions. The thrust bearing, with associated upper and lower bearing races, is disposed between these four shoulders. When the bearing section is subjected to on-bottom loading, the axial thrust loads react against the thrust bearing through the outer upper shoulder and the inner lower shoulder of the containment chamber. Under off-bottom loading, the axial thrust loads react against the thrust bearing through inner upper shoulder and the outer lower shoulder.
One problem with the Wenzel bearing assembly is that under both on-bottom and off-bottom loading conditions, the thrust bearing is always being loaded eccentrically, due to the radial offset between the resultant load paths acting through the operative shoulders of the bearing containment chamber. Such eccentric loading tends to twist the bearing races, which can result in damage to the bearing assemblies and reduced bearing service life. Eccentric loading thus reduces the total thrust load that a given thrust bearing can withstand, and necessitates the use of more robust thrust bearing components than would be required if the thrust loads were transferred through the thrust bearing without eccentricity.
In practical terms, this is primarily a concern with respect to on-bottom thrust loads, the highest of which are typically much greater than the highest off-bottom loads for a given drilling operation. Therefore, a bi-directional thrust bearing will typically be designed for the maximum on-bottom thrust load. Since the bi-directional thrust bearing will typically have the same maximum thrust load capacity in both directions, and since the maximum off-bottom thrust loads will be substantially less than the maximum on-bottom thrust loads, the expected off-bottom thrust loads will rarely if ever be a limiting design factor.
For at least the foregoing reason, there is a need for a bearing assembly for a downhole motor in which on-bottom thrust loads are transferred through bi-directional thrust bearings without eccentricity, thus maximizing the on-bottom thrust load capacity of the bearing assembly. The embodiments described herein are directed to this need.